1.3.1.5 Changing the font

AUCTeX provides convenient keyboard shortcuts for inserting macros
which specify the font to be used for typesetting certain parts of the
text. They start with C-c C-f, and the last C- combination
tells AUCTeX which font you want:

C-c C-f C-b

Insert bold face ‘\textbf{∗}’ text.

C-c C-f C-i

Insert italics ‘\textit{∗}’ text.

C-c C-f C-e

Insert emphasized ‘\emph{∗}’ text.

C-c C-f C-s

Insert slanted ‘\textsl{∗}’ text.

C-c C-f C-r

Insert roman \textrm{∗} text.

C-c C-f C-f

Insert sans serif ‘\textsf{∗}’ text.

C-c C-f C-t

Insert typewriter ‘\texttt{∗}’ text.

C-c C-f C-c

Insert SMALL CAPS ‘\textsc{∗}’ text.

C-c C-f C-d

Delete the innermost font specification containing point.

If you want to change font attributes of existing text, mark it as a
region, and then invoke the commands. If no region is selected, the
command will be inserted with empty braces, and you can start typing the
changed text.

Most of those commands will also work in math mode, but then macros like
\mathbf will be inserted.